1. Field of the Invention
The present invention generally relates to a light emitting device (electroluminescence device) that emits light when an electric field is applied and more particularly to an organic electroluminescence device with improved light emitting efficiency.
2. Description of the Related Art
Recently, organic EL has been drawing attention for use in illumination, display light sources, and the like. Light emitting materials used for organic EL, however, have a problem of low durability, which makes the organic EL difficult to be put into practical use.
It is known that organic materials inherently remain in exited state for a long time, whereby the chemical bonding of the materials is broken and the light emission performance is degraded with time. This low durability is a big challenge in employing an organic substance to a light emitting device.
Typically, organic EL devices have a structure in which an electrode layer and a plurality of organic layers are laminated on a substrate and light emitted from a light emitting layer is outputted through a transparent electrode. In this structure, the light incident on each interface between the layers on the light output side at an angle greater than the critical angle is totally reflected back and contained inside of the device, thereby unable to extract the light to the outside. Consequently, it is difficult to efficiently extract the emission light, and it is said that the light extraction efficiency is about 20% for a transparent electrode having a refractive index of ITO or the like which is being used commonly as a material of transparent electrode.
U.S. Patent Application Publication No. 20070114523 proposes a technique for improving the light extraction efficiency of an organic EL device by disposing a scattering layer which includes metal fine particles inside of the device and scattering the emission light.
In the mean time, J. R. Lakowicz et al., “Radiative decay engineering. 2. Effects of Silver Island Films on Fluorescence Intensity, Lifetimes, and Resonance Energy Transfer”, Analytical Biochemistry, Vol. 301, Issue 2, pp. 261-277, 2002 describes that the exciton lifetime of a dye placed adjacent to a metal fine particle is reduced and the durability is improved. In relation to this, W. Li et al., “Emissive Efficiency Enhancement of Alq3 and Prospects for Plasmon-enhanced Organic Electroluminescence”, Proc. of SPIE, vol. 7032, pp. 703224-1-703224-7, 2008 proposes a method for enhancing emission of an organic light emitting device by disposing an island shaped metal near a light emitting layer. This emission enhancement is due to the fact that the dipole radiation from the light emitting device induces a surface plasmon (local plasmon) on the metal surface and energy is absorbed which is then reradiated as a new emission. That is, a new emission transition induced by the plasmon is added to the original emission process of the light emitting device, whereby advantageous effects of reducing the upper level lifetime (radiactive lifetime) may be obtained. In this way, it is expected that the utilization of plasmon resonance may provide advantageous effects of improving the durability of the light emitting device through radiactive lifetime reduction, as well as improving the light emission efficiency.
Further, A. Fujiki et al., “Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode”, Applied Physics Letters, Vol. 96, pp. 043307-1-043307-3, 2010 reports that, in an EL device, plasmon enhancement effect was obtained for a light emitting material having a very low quantum efficiency value of 5.0×10−8 to 2.0×10−8 by disposing Au fine particles with a particle diameter of 12 nm adjacent to a transparent electrode (ITO) used as the anode of the device.
But, in W. Li et al., “Emissive Efficiency Enhancement of Alqa and Prospects for Plasmon-enhanced Organic Electraluminescence”, Proc. of SPIE, vol. 7032, pp. 703224-1-703224-7, 2008, the emission enhancement due to the plasmon enhancement effect is confirmed only for photoexited light emitting devices (photoluminescence devices (PL devices)), and no report of successful example is found for field exited EL devices.
Further, A. Fujiki et al., “Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode”, Applied Physics Letters, Vol. 96, pp. 043307-1-043307-3, 2010 describes that higher plasmon enhancement effect was obtained for lower quantum efficiency. It is generally known that the plasmon enhancement effect can be obtained in a case where the quantum efficiency is very low and A. Fujiki et al., “Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode”, Applied Physics Letters, Vol. 96, pp. 043307-1-043307-3, 2010 does not describe plasmon enhancement effect for a material of high quantum efficiency.